1. Technical Field
The present invention generally relates to trunk piston engine lubricating oil compositions.
2. Description of the Related Art
Trunk piston engines operate using various types and qualities of diesel fuels and heavy fuel oils. These fuels typically contain high concentrations of asphaltenes, generally the heaviest and most polar fraction of petroleum distillate. Asphaltenes are highly complex compounds believed to be composed of polyaromatic sheets containing alkyl side chains, and are generally insoluble in lubricating oils. When heavy fuel oils and conventional lubricant oil compositions mix in different temperature regions of a trunk piston engine, black sludge (such as asphaltene deposits or other deposits) and other asphaltene derived deposits (such as undercrown deposits) tend to form. The formation of black sludge or deposit can adversely affect the service interval and maintenance cost of the trunk piston engine.
Presently, there is a move in the industry in different regions of the world to replace Group I base oils with Group II base oils in trunk engine oils. Group II base oils generally have a lower aromatic content than Group I base oils, thereby resulting in a loss of heavy fuel oil (also known as residual fuel oil) compatibility when Group II or higher base oils are used in trunk piston engine lubricating oils rather than Group I base oils. It is believed that this loss of heavy fuel oil compatibility is due to the much lower solubility of asphaltenes in the Group II or higher base oils compared to Group I base oils. Generally, the problem of the loss of heavy fuel oil compatibility has been typically addressed by increasing the amount of detergent-containing trunk piston engine lubricating oil additive packages.
U.S. Patent Application Publication No. 20080039349 (“the '349 application”) discloses a lubricating oil composition containing (a) an oil of lubricating viscosity; (b) at least one overbased metal detergent; and (c) at least one substituted diaryl compound. The '349 application further discloses that the lubricating oil composition exhibits improved asphaltene dispersancy in a trunk piston diesel engine.
U.S. Patent Application Publication No. 20090093387 (“the '387 application”) discloses a lubricating oil composition containing (a) a Group II basestock, and (b) a neutral or overbased metal hydrocarbyl-substituted hydroxybenzoate detergent having a basicity index of less than 2. The '387 application further discloses that the neutral or overbased metal salicylate detergent having a basicity index of less than 2 improves asphaltene dispersancy in Group II basestocks.
WO2008102114 (“the '114 application”) discloses a liquid lubricant base oil composition useful for a 2-stroke marine diesel engine cylinder oil, a 2-stroke marine diesel engine system oil, and a 4-stroke marine diesel engine. The lubricant base oil composition disclosed in the '114 application contains (a) a base stock comprising at least 95 wt. % saturated hydrocarbons, and (b) 0.2 to 30 wt. % of an aromatic (brightstock) extract. A bright stock is a high viscosity base oil which has been conventionally produced from residual stocks or bottoms and has been highly refined and dewaxed. The '114 application further discloses that the combination of a Group II base oil and a low polycyclic aromatic brightstock extract demonstrated improved viscosity ratio and improved oxidation and wear performance.
It would be desirable to develop a trunk piston engine lubricating oil composition containing a base stock containing at least 90% by weight saturated hydrocarbons, which exhibits improved heavy fuel oil compatibility.